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Appendix VI.

1031

Back in 1877, or thereabouts, when these wheels were being first
introduced, the binding material of the emery was so unsatisfac-
tory as to cause glazing by not falling away with the wear of the
emery particles, and the stones often broke by centrifugal force
due to insufficient strength in the material. The wheels of the
present day are a great advance on these former ones, and some
notion of this improvement may be gained when it is stated that
TV in. thickness of wheel may be adopted without breaking. All
emery wheels were formerly made from corundum or from emery
powder, or both (amorphous conditions of the sapphire and
ruby), but some very valuable artificial products are now on the
market in addition to the natural substances. Carborundum is
one of these, consisting of salt, sawdust, and coke subjected to
great heat in electric furnaces for 24 hours, the crystals produced
being ground and sifted. For use it is mixed with kaolin and
Jelspar, and then baked into wheels. This material is suited
for grinding hard cast iron and mild steel, but for hardened steel
the older emery appears to be the best, the latter being pressed
into moulds and baked with some binding material that is itself
abrasive, and will therefore wear away at the same rate as the
grinding material. The harder the work to be ground, the
coarser and softer the wheel should be, and the surface speed
should be about 6000 ft. per min. for mild steel, 4000 to 5000
for hard steel, and 4000 for cast iron. It is most important that
there should be a plentiful supply of cold water (not soda) to the
grinding point, which is to prevent the softening action due to
the heat caused in grinding, and it is also unwise to use the same
water over and over again, as it becomes charged with grease and
glazes the wheel. A greater truth and better finish also results
in the use of water, but the best possible effect is produced when
a little lapping is finally done on the job, thus removing the
grinding marks and giving great accuracy. If water cannot be
fed to the wheel, as is sometimes the case with internal grinding,
it should be poured on the outside of the work. All long work
should be well steadied, or untruth will certainly occur through
springing, and a roughing down to approximate size should always
precede the actual grinding, so as to leave as little as possible for
the emery wheel to do. It must be realised that although a very